The multiplicity, tissue specificity and ontogenetic patterns of expression of 33 gene loci, encoding 16 enzyme and isozyme systems, will be examined in a series of five species of teleostean fishes in the genus Barbus and in their interspecific hybrids by acrylamide gel electrophoresis. Preliminary studies conducted in this laboratory have shown these fishes to be ideally suited to the investigation of gene expression and regulation, and will permit us to gain valuable insight into the regulatory mechanisms underlying differential gene expression.